In the aerospace domain, composite materials are being used more and more to reduce the mass of an aircraft. A composite material component comprises fibres, for example made of carbon, that are embedded in a matrix of resin, for example thermosetting resin.
In one mode of operation, firstly, a preform of fibres, preferably pre-impregnated fibres, is produced and subsequently polymerized in particular in an autoclave so as to obtain the final component.
In order to obtain the preform of fibres, layers of fibres are stacked successively one on top of another, the fibres being oriented in at least one predetermined direction for each layer depending on the mechanical characteristics that are desired.
In order to increase productivity, automatic machines such as those referred to as ATL (Automated Tape Laying) machines are used to produce the stack of layers of fibres.
According to one embodiment, the layers of fibres are cut out of a strip of fibres stored in the form of a reel, the strip of fibres being pressed against a support in the form of a film of paper or plastics material that has the role of a separator when the strip is wound on a reel.
The absence of undulations in the fibres is an important criterion for the quality of the final component.
The undulations in the fibres may originate from the layers of fibres or be generated during the stacking of the layers or during polymerization.
Thus, prior to being placed in position, the layers of fibres can have stresses which tend to generate the undulations of fibres in the layers of fibres.
For example, during the production of the strip of fibres, when the support is in the stretched state at the time when the fibres are placed thereon, the releasing of the extension of the support will bring about compression of the fibres, which will undulate and remain in this state in the preform of fibres.
According to another aspect, with the strip of fibres wound on a reel, there is a difference in tension between the fibres close to the internal surface and those close to the external surface, resulting in there being compressed fibres in the region of the internal surface of the strip of fibres wound on a reel. This compressed state of the fibres in the region of the internal surface is even greater, the thicker the strip and the smaller the winding radius. In the case of a strip of fibres wound on a reel and close to its expiration date, the resin that impregnates the fibres is “drier” such that the fibres close to the internal surface tend to retain their undulations during the stacking of the layers of fibres.
There are no non-destructive tests that ensure the absence of undulations in fibres. The only test that is possible consists in cutting up a finished component into localized sections in order to observe the layers of fibres under a microscope.
However, this test phase is only possible at the end of the method for manufacturing the components. Consequently, if the layer of fibres has a defect prior to stacking, this is only detected at the end of the method and the steps of depositing layers and polymerization are carried out in vain.